The invention relates to underground coal gasification and more particularly to an in situ method for the gasification of deep, thin coal seams.
The United States and the world are facing a severe energy crisis, caused largely by the shortage and political instability of the world petroleum supply. Coal is the most abundant fossil fuel in the United States, with sufficient reserves to last for hundreds of years. About 400 billion tons can be easily mined; an estimated 6.4 trillion tons lie too deep to mine economically. Coal seams near the surface are generally recovered by strip mining. However, while the coal is abundant, the use of coal as fuel presents serious environmental problems.
Coal gasification methods have been used to produce combustible fuel from coal. When coal is heated in the presence of oxygen and steam it gives off a mixture of combustible gases which can be refined and purified and used as fuel. These product gases are environmentally more advantageous than coal since they burn cleaner, producing less air pollution and are easier to transport. Above-ground gasification processes are available but still require expensive and difficult mining and transportation before surface processing.
It is more advantageous to gasify coal in situ by chemically reacting the coal underground to produce combustible product gases. An oxidizing gas and steam are pumped down through an injecting well and the coal is ignited. The product gases are removed through a production or recovery well. For the process to occur, a permeable path through the coal must be provided between the injection and production well to permit the high volume gas flow that is required.
The most viable linking methods include countercurrent or reverse combustion, directional drilling, and electrolinking. Countercurrent or reverse combustion linking is the most commonly used technique for enhancing the permeability of a coal bed. Air is forced into the injection well and flows to the production well through natural fissures in the coal bed. The coal at the bottom of the production well is ignited and a burn front is drawn by conduction toward the source of oxygen, charring a narrow channel countercurrent to the flow of air. The directional drilling method produces a gasification channel in the coal by drilling along a coal steam at varying angles and intersecting the production and injection wells. The electrolinking method utilizes an electric current to char a channel of coal between two access holes.
Most of the coal in the western United States is found in thick seams for which the reverse combustion and directional drilling methods of linking are more reliable and more economical. A link is established at the bottom of the thick coal seam so that as the gasification process progresses coal falls into the void, producing coal rubble with a large surface area for coal-gas reactions. It is estimated that 1.8 trillion tons of coal can be reached by conventional underground coal gasification technology. This represents a tremendous resource that cannot generally be reached by mining.
However most of the coal in the eastern United States is located in deep, thin seams for which the conventional in situ coal-rubblization process for achieving intimate contact between the gases and coal in order to produce high quality, stable composition gas cannot be utilized. The seams are not thick enough to produce significant amounts of falling coal and eastern bituminous or swelling coals do not flake and fall like western lignite and subbituminous or shrinking coals. An additional coal resource that has not been utilized is the coal reserves left at abandoned strip mines, particularly in the eastern and southwestern United States. At numerous abandoned strip mines relatively thin coal seams were surface mined into a hillside or down a modest dip until the overburden became so thick that further mining was not economical. Often the mines were not backfilled so the coal face remains exposed over long distances. Sometimes the coal was mined a few hundred feet further into the seam by using augers.
It is an object of the invention to provide a method for the gasification of deep, thin coal seams.
It is another object of the invention to provide a method to gasify coal seams in abandoned strip mines from the exposed coal faces.
It is also an object of the invention to provide a method for achieving intimate contact between gases and coal in a thin seam during an in situ gasification process.